1. Field of the Invention
This invention relates to a combustion chamber structure for an engine and, more particularly, to a combustion chamber structure in which a "swirl effect" of intake air is utilized in order to promote combustibility in a combustion chamber.
2. Description of Related Art
It has been known to provide a swirl effect, i.e., an induction turbulence effect, to intake air supplied to an engine in order to improve combustion performance of the engine. An intake port which opens at a combustion chamber formed in a cylinder head has a shape which is properly designed so as to let the intake air, introduced from the intake port, swirl in the combustion chamber during an intake stroke of the engine for promotion of mixture of fuel and air. In such a combustion chamber structure, the intake port for producing a swirl flow of the intake air (which is hereinafter referred to as a "swirl port") is generally provided so that a center line of the swirl port points in a tangential direction of a cylindrical inner surface of the cylinder. Such an intake port construction is described in, for example, Japanese Utility Model Publication No. 52-32,812.
However, the combustion chamber of this publication can cause a problem in that intake air flowing from the swirl port into the combustion chamber, in particular, intake air which is directed towards the periphery of the combustion chamber, is interfered with by a wall surface of the combustion chamber. As a result, production of the swirl flow is impaired.
Typically, an intake valve and an exhaust valve are operated at a proper timing so as to introduce intake air into the cylinder and discharge burned gases from the cylinder. Such intake and exhaust valves have "conical-frustrum" valve heads, which are otherwise well known as poppet type or mushroom type valve heads, on tip ends of their valve stems. These valves are reciprocally moved in straight lines, respectively, so as to open and close the intake and exhaust ports. In an engine of the type described in, for example, Japanese Patent Publication No. 59-51,647, a downstream portion of an intake port is curved to open towards an inner face of the cylinder so as to cause a swirl flow of intake air, in particular, when the engine operates at low loads wherein only a small amount of intake air is introduced into the cylinders, thereby promoting mixing of fuel and air and improving the combustibility or output performance of the engine.
In an engine having such poppet type intake valves in intake ports, the conical-frustum valve head of an intake valve tends to obstruct the flow of intake air when the intake air flows from the opening of the intake port into the cylinder during the intake stroke, increasing resistance to the intake air. Such an increase in resistance may possibly result in impeding an increase in air charging efficiency. Moreover, curving the intake port so as to produce a swirl flow, i.e., an induction turbulence, of the intake air in the cylinder breaks the force of flow of the intake air into the cylinder, so that a suitable swirl flow is not produced.
For the purpose of providing a more detailed description that will enhance an understanding of the operation of a poppet type valve such as that described above, reference is made to FIGS. 1 and 2. In an engine constructed as shown in FIG. 1, intake air, which is introduced into a cylinder 33 through an intake port 34 while an intake valve 31 is opened, impinges on a valve face of a conical-frustum valve head 32 of the intake valve 31, so that its direction of flow is changed from an obliquely downward direction along the intake port 34 to an approximately horizontal direction. Owing to the directional change, it is hard to smoothly introduce the intake air into the cylinder 33, resulting in an increase in resistance to its flowing in the cylinder.
Even if poppet types of intake valves 41 and exhaust valves 43 are designed with angles which properly allow intake air to flow in through the intake port 42 and pass over the conical-frustum valve head of the intake valve 41 without changing its direction of flow as shown in FIG. 2, depending upon the relation between the direction of intake air flow and an angle of inclination of the exhaust valve 43 with respect to the intake port 42, there is caused an impingement of the intake air, passed over the conical-frustum valve head of the intake valve 41, on an under surface the exhaust valve 43. In such a case, it is hard to smoothly introduce the intake air into the cylinder 44, so as to increase its flowing resistance with respect to the cylinder.